hi i am not wanting to make this it is a question i need answerd its been stuck in my head for weeks but what would happen if on a combustion gun right after the reducer there was say a ball valve(or any valve) you close the valve put a potato down the barrel open the back and put fuel in close the back and hit the sparker what would happen would it turn into some pneumatic gun so all you do is open the valve and the potato shoots out

The simple answer is no, it will not work as you intend. The pressure from the combustion is short lived. Wait for someone else to explain the science.

I've had something similar happen with my combustion launcher. Overloading the barrel with a bunch of rocks (too heavy) or even dipping the first few feet of the barrel in the pool. Once fired I believe it creates a vacuum, from the sound of it, and the end cap requires a wrench to get back off.

RBurke83-
Not likely. I believe a contained explosion produces slightly more pressure, then a successful launch. Reason being the peak pressure is never reached because of the chamber volume increasing as the projectiles moves down the barrel and eventually exits.

Short answer: Roughly one second after ignition the pressure in a closed combustion chamber is basically back at atmospheric pressure. To get any performance out'a the gun you'll need to get the valve open in much less than 1 second after igniting the fuel.